Diesel particulate filter overstress mitigation

ABSTRACT

Methods and systems for reducing overstress of a DPF are provided herein. One example of such a method is based on radial temperature gradient near the exit face of the diesel particulate filter. In this way, it becomes possible to more closely correlate radial stresses to actual risk of DPF fracture.

BACKGROUND AND SUMMARY

Temperature non-uniformity within a diesel particulate filter (DPF)occurs during filter regeneration. This non-uniformity is a growingconcern because it has become especially large with the filters usedwith modern diesel engines with low engine-out levels of the nitrogenoxides. In the past, those nitrogen oxide gases have served to passivelyoxidize carbon soot and thereby remove the accumulated soot. With themodern engines, the diesel soot accumulates in the filter until anactive step is taken to initiate soot combustion, beginning an activeregeneration, to burn off most of the soot within a relatively shorttime interval, and creating more severe temperature non-uniformity. Evenwith the choice of low coefficient of thermal expansion ceramics, suchlarge temperature non-uniformity from an active regeneration can createinternal stresses that are sufficiently large to fracture the filterceramics. In DPFs of the single-brick monolithic form that is preferredfor large-volume manufacturing and economy, fracture across the monolithcauses a sudden, catastrophic, and irreversible loss of filtrationperformance. This uncertainty about maintenance of filtering functionwith monoliths has inhibited their wider use. Indeed this risk formonoliths has led to wide commercial use of multi-segment mortaredstructures, in spite of the associated high material and manufacturingcosts.

Thus, to reduce the risk of sudden failure and to extend the workinglife of an economical monolithic DPF that is subjected to multipleregenerations, one would directionally seek to reduce the temperaturenon-uniformity in the DPF during regeneration. More particularly, onewould reduce the non-uniformity in a way that lowers the stress in theceramic to a level below some particular level that related to thestrength of the ceramic and its coefficient of thermal expansion. In aresearch environment, this particular relationship can be developed bymeans of finite element analysis based on temperatures detected by afull field of tens of temperature sensors. To simplify the calculation,the filter ceramic body can be adequately approximated as a continuummaterial with a relatively simple type of non-isotropy, namely, one withphysical properties generated from isotropic properties of the honeycombwall materials in the particular geometry of the particular honeycomb.With such finite element calculations carried out at a series of timepoints during a regeneration, one may estimate the moment-by-momentinternal stresses within the DPF and the corresponding probability offailure. This approach is suited for research.

However, the applicant herein recognizes that placing tens of sensorsthroughout a DPF is costly and will not be practical outside of aresearch and advanced development environment. Likewise, having a lessappropriate estimator will either keep the in-use stresses lower thannecessary or will lead to failures. If the stresses are kept too low,the operation leads to inefficiencies in the amount of fuel used toinitiate more particulate filter regenerations than otherwise needed. Ifthe less appropriate estimator errs on the other side, more failureswill occur. Without having an appropriate simple estimator, theparticulate filter might have to be designed for tolerance of fracturethrough the use of multi-segment mortared structures, but this approachincreases manufacturing costs unnecessarily and serves as a concern forreduced emission reduction performance characteristics over time.

As such finite element methods are cumbersome, simplified methods andsystems for preventing the stress of a DPF are provided herein. Byestimating the stress from a small number of temperature sensors, onecan use more generally the methods to take action to prevent the stressfrom rising above a critical level for unacceptably high probability offracture. Although these methods and systems are intended to apply tothe more economical monoliths, these methods and systems also apply tomortared structures, wherein the mortar (as manufactured or aged) allowsa crack to pass through to the next segment without much deflection, asif it were a monolith.

One example of such a simplified method includes measuring a radialtemperature gradient near the periphery near the exiting-flow face ofthe diesel particulate filter, and adjusting at least one engineoperating parameter to control the radial temperature gradient, asindicated by the measured radial temperature gradient at one angularposition near the exit face of the diesel particulate filter. Anembodiment of this example of such a system includes two near-exit-facetemperature sensors configured to measure a radial temperature gradientnear the periphery near the exit face of the diesel particulate filterand a controller configured to adjust at least one engine operatingparameter based on the measured temperature gradient to limit thestress, as calculated assuming that the same gradient extends deeplyinto the DPF from the exit face.

Specifically, during an actively initiated DPF regeneration at exhaustflows near idle-engine exhaust gas flow, the high temperature gradientsand the high absolute temperature occur through much of the length ofthe filter. In calculating the stress in the ceramic, the radial thermalgradient is integrated inward toward the center along a line parallel tothe flow axis, beginning a value of zero at a chosen free surface,namely, the exit face. When the radial gradient is high through much ofthe half-length of the filter, the integral is large.

Therefore, it is preferred to have the highest gradients occur lessgenerally within the length of DPF, namely, and preferably, mostly nearthe exit face and away from the center of the DPF during filterregeneration. In this way, the integral that is calculated is not aslarge as before, although the local radial gradient near the end facemay be the same or higher than in the former case. This localization ofthe highest gradients may be achieved by design by controlling variousengine operating parameters, such as with increased air flow, to movethe highest temperatures and gradients to the rear of the DPF. By doingso, the integrated quantity, stress, as experienced by the DPF islowered. However, the pair of temperature sensors that are both locatednear the end face is not able to sense the difference that has causedthe high gradients to be concentrated near the exit face, thereforeleading to a calculation of stresses that are likely to be higher thanactual stresses, resulting in unnecessary inefficiency in fuel usage, inthe effort to keep the DPF filter from being at risk of fracturing.

The systems and methods provided herein may help to correlate thesestresses experienced by the DPF to those estimated with a small numberof sensors, which include the two sensors near the exit face, for theradial temperature gradient measurement, as above, near the exit facetogether with an additional sensor to indicate how deeply into thefilter the high temperature extends. In such way, it becomes possible tomore closely correlate stresses to actual risk of DPF degradation, inturn, allowing better adjustments of the various DPF regenerationparameters and/or other engine operating parameters to reduce stressesexperienced by the DPF during regeneration only as much as is necessaryto prevent unacceptable risk of fracture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example engine including a DPF overstress mitigationapparatus.

FIG. 2 illustrates an example DPF overstress mitigation apparatus.

FIG. 3 illustrates another example DPF overstress mitigation apparatus.

FIG. 4 is a flowchart illustrating a method for avoiding DPF overstressusing a DPF overstress mitigation apparatus.

FIG. 5 is a schematic diagram illustrating temperature profile of a DPFduring regeneration.

FIG. 6 is a schematic diagram illustrating the fracture causing radialstress resulted from temperature non-uniformity in a DPF during filterregeneration.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram showing one cylinder of multi-cylinderengine 10, which may be included in a propulsion system of anautomobile. Engine 10 may include a diesel compression ignition engine.Engine 10 is controlled at least partially by a control system includingcontroller 12 and by input from a vehicle operator 132 via an inputdevice 130. In this example, input device 130 includes an acceleratorpedal and a pedal position sensor 134 for generating a proportionalpedal position signal PP. Combustion chamber (i.e. cylinder) 30 ofengine 10 includes combustion chamber walls 32 with piston 36 positionedtherein. Piston 36 is coupled to crankshaft 40 so that reciprocatingmotion of the piston is translated into rotational motion of thecrankshaft. Crankshaft 40 may be coupled to at least one drive wheel ofa vehicle via an intermediate transmission system. Further, a startermotor may be coupled to crankshaft 40 via a flywheel to enable astarting operation of engine 10.

Combustion chamber 30 receives intake air from intake manifold 44 viaintake passage 42 and exhausts combustion gases via exhaust passage 48.Intake manifold 44 and exhaust passage 48 can selectively communicatewith combustion chamber 30 via respective intake valve 52 and exhaustvalve 54. In some embodiments, combustion chamber 30 may include two ormore intake valves and/or two or more exhaust valves.

Fuel injector 66 is shown coupled directly to combustion chamber 30 forinjecting fuel directly therein in proportion to the pulse width ofsignal FPW received from controller 12 via electronic driver 68. In thismanner, fuel injector 66 provides what is known as direct injection offuel into combustion chamber 30. The fuel injector may be mounted in thetop of the combustion chamber or in the side of the combustion chamber,for example. Fuel is delivered to fuel injector 66 by a fuel system (notshown) which typically includes a fuel tank, a fuel pump, and a fuelrail. In some embodiments, combustion chamber 30 may alternatively oradditionally include a fuel injector arranged in intake passage 44 in aconfiguration that provides what is known as port injection of fuel intothe intake port upstream of combustion chamber 30.

Intake passage 42 may include a throttle 62 having a throttle plate 64.In this particular example, the position of throttle plate 64 is variedby controller 12 via a signal provided to an electric motor or actuatorincluded with throttle 62, a configuration that is commonly referred toas electronic throttle control (ETC). In this manner, throttle 62 isoperated to vary the intake air provided to combustion chamber 30 amongother engine cylinders. The position of throttle plate 64 may beprovided to controller 12 by throttle position signal TP. Intake passage42 may include a mass air flow sensor 120 and a manifold air pressuresensor 122 for providing respective signals MAF and MAP to controller12.

Exhaust gas sensor 126 is shown coupled to exhaust passage 48 upstreamof emission control device 70. Sensor 126 may be any suitable sensor forproviding an indication of exhaust gas air/fuel ratio such as a linearoxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), atwo-state oxygen sensor or EGO, a HEGO (heated EGO), a NOx, HC, or COsensor. Emission control device 70 is shown arranged along exhaustpassage 48 downstream of exhaust gas sensor 126. Device 70 may include alean NOx trap, selective catalytic reduction (SCR) catalyst, particulatefilter such as a diesel particulate filter (DPF), a three-way catalyst(TWC), various other emission control devices, or combinations thereof.Specifically, device 70 includes a catalyzed diesel particulate filterhaving a ceramic substrate.

In some embodiments, during operation of engine 10, emission controldevice 70 is periodically reset. For example, a diesel particulatefilter is regenerated periodically by burning off soot accumulated inthe diesel particulate filter.

Controller 12 is shown in FIG. 1 as a microcomputer, includingmicroprocessor unit 102, input/output ports 104, an electronic storagemedium for executable programs and calibration values shown as read onlymemory chip 106 in this particular example, random access memory 108,keep alive memory 110, and a data bus. Controller 12 receives varioussignals from sensors coupled to engine 10, in addition to those signalspreviously discussed, including measurement of inducted mass air flow(MAF) from mass air flow sensor 120; engine coolant temperature (ECT)from temperature sensor 112 coupled to cooling sleeve 114; a profileignition pickup signal (PIP) from Hall effect sensor 118 (or other type)coupled to crankshaft 40; throttle position (TP) from a throttleposition sensor; and absolute manifold pressure signal, MAP, from sensor122. Engine speed signal, RPM, is generated by controller 12 from signalPIP. As described above, FIG. 1 shows only one cylinder of amulti-cylinder engine, and that each cylinder may similarly include itsown set of intake/exhaust valves, fuel injector, glow plug, etc.

FIG. 2 and FIG. 3 illustrate examples of DPF overstress mitigationsystems 200 for avoiding mechanical overstress of a diesel particulatefilter caused by temperature non-uniformity during regeneration of theDPF. Such overstress may lead to fracture of the DPF filter ceramics,which in turn will lead to loss of DPF filtration performance. For thesake of convenience, similar parts are labeled similarly in FIGS. 2 and3.

An embodiment of the system 200 includes at least two temperaturesensors 204 located near the periphery and close to the exit face 206portion of the DPF. The temperature sensors 204 are radially spaced fromeach other (and may be slightly longitudinally displaced from oneanother in a direction of exhaust flow) for measuring a radialtemperature gradient

$\left( \frac{\mathbb{d}T}{\mathbb{d}r} \right)$near the exit face. The system 200 also includes a controller 208configured to adjust at least one engine operating parameter to providefeedback control of the probability of failure, which is linked to theradial temperature gradient

$\left( \frac{\mathbb{d}T}{\mathbb{d}r} \right)$more simply when the equi-temperature contours are cylindrical, forexample, when the radial temperature gradient

$\left( \frac{\mathbb{d}T}{\mathbb{d}r} \right)$is greater than a maximum allowable radial temperature gradient

$\left( {{MAX}\frac{\mathbb{d}T}{\mathbb{d}r}} \right)$based on cylindrical equi-temperature contours.

${MAX}\frac{\mathbb{d}T}{\mathbb{d}r}$may be a predetermined value. Controller 208 may be included incontroller 12, or may be another controller in a control system separatefrom controller 12.

$\left( {{MAX}\frac{\mathbb{d}T}{\mathbb{d}r}} \right)$

The maximum allowable radial temperature gradient is a temperaturegradient below which mechanical stress experienced by the DPF is lessthan a maximum allowable stress (MAXσ) for the volume of material underthat high stress, so that an acceptably-low probability of DPF failureis achieved. In some examples, the maximum allowable radial temperaturegradient near the exit face is based on the locations of the exit-facetemperature sensors that are used to measure the radial temperaturegradient near the exit face, the material and cellular geometry of thefilter, and the diameter and length of the DPF.

Further, the maximum allowable radial temperature gradient near the exitface may vary with an operating condition of the diesel particulatefilter and/or other engine operating parameters, such as air/fuel ratio,exhaust oxygen concentration, velocity of exhaust flow through thediesel particulate filter, the number of times the filter has beenregenerated, regeneration frequency, filter age, and/or mechanicalproperties of the material that constitutes the filter together with anycatalyst thereon. For example, if the mechanical strength of the filteris low due to the age of the filter, a large number of regenerationcycles has been performed, and/or the material constituting the filterhas been compromised, the maximum allowable temperature gradient may beset lower to reduce the probability of fracture of the filter ceramicmaterial.

In one specific example, the maximum allowable radial temperaturegradient measured at 1 inch in from the exit face 206 and between 1 cmand 2 cm from the periphery 207 of the DPF is set to approximately 175°C./cm.

In some examples, the system 200 may also include a deep temperaturesensor 214 for measuring a deep temperature (T_(d)) shown in FIG. 3. Thedeep temperature sensor 214 is positioned away from the exit face 206and deeper into the diesel particulate filter 202 longitudinally alongthe direction of overall exhaust flow to a further upstream positionthan the radial temperature gradient sensors 204 with a radial positionsimilar to the sensor pair. This deep sensor allows estimation ofwhether the equi-temperature contours within a regenerating DPF are of amore-conical shape or of a more-cylindrical shape. In these examples,the controller 208 may be further configured to adjust the at least oneengine operating parameters based further on the measured deeptemperature (T_(d)). The controller 28 may for example modify themaximum allowable temperature gradient

$\left( {{MAX}\frac{\mathbb{d}T}{\mathbb{d}r}} \right),$based on the measured deep temperature (T_(d)). In one particularexample, the maximum allowable temperature gradient

$\left( {{MAX}\frac{\mathbb{d}T}{\mathbb{d}r}} \right),$in the form of

${{{MAX}\frac{\mathbb{d}T}{\mathbb{d}r}}}_{CONECONTOURS}$may be calculated using the following equation:

$\left. {{{{MAX}\frac{\mathbb{d}T}{\mathbb{d}r}}}_{CONECONTOURS} = {{\quad{{MAX}\frac{\mathbb{d}T}{\mathbb{d}r}}}_{CYLINDERCONTOURS}\left\{ {1 + \left( \frac{\left. {\mathbb{d}\left( {\frac{\mathbb{d}T}{\mathbb{d}r}/\frac{\mathbb{d}T}{\mathbb{d}r}} \right._{o}} \right)}{{\mathbb{d}\sigma}/\sigma_{o}} \right._{CYL}} \right)\left( \frac{1000{^\circ}\mspace{14mu}{C.\;{- \; T_{d}}}}{1000{^\circ}\mspace{14mu}{C.\;{- \; 600}}{^\circ}\mspace{14mu}{C.}} \right)}} \right\}$

${{{MAX}\frac{\mathbb{d}T}{\mathbb{d}r}}}_{CYLINDERCONTOURS}$

Where is the maximum allowable radial temperature gradient by finiteelement analysis (FEA) with cylindrical equi-temperature contours. Themiddle term is the instantaneous slope of the relation of the radialgradient to the stress in FEA calculations with cylindricalequi-temperature contours. In one example,

${{{MAX}\frac{\mathbb{d}T}{\mathbb{d}r}}}_{CYLINDERCONTOURS}$is approximately equal to 175° C./cm measured at 1 inch in from the exitface and 1 cm and 2 cm from the periphery 207 of the DPF to achieve aprobability of failure of less than 0.002.

Where T_(d) is the deep temperature, taken as the temperature read at3.66 inch in from the exit face 206 and 2 cm from a periphery wall 207of the 8 inch diameter by 10 inch length DPF. In this case, the middleterm is 0.7 (dimensionless) for the instantaneous slope of the relationof the radial gradient to the stress in FEA calculations withcylindrical equi-temperature contours.

In some examples, the signal detected by the deep temperature sensor 214is used to modify an approximate temperature difference signal of theradial gradient temperature sensors 204. The modification can also becarried out via an analog circuit approximating algorithm, adifferential thermocouple, such as one that includes two hotthermocouple junctions 204 in series with one reversed, and anappropriately calibrated resistor.

The controller 208 is coupled to various sensors 210 for sensing variousengine operating conditions and actuators 212 for controlling parametersof the engine operation. Examples of the sensors 210 and actuators 212are listed and described in reference to FIG. 1.

The at-least-one engine operating parameter adjusted by the controller208 for controlling the DPF temperature non-uniformity duringregeneration may include, for example, throttle position for adjustingair flow, DPF regeneration frequency, exhaust oxygen concentration, lateinjection amount, etc. For example, the controller 208 may increaseairflow to the DPF to help to cool DPF and push the peak temperature inthe DPF during regeneration further towards the exit face of the DPF todecrease the stress experienced by the DPF and the associatedprobability of failure. Additionally, or alternatively, the controller208 may increase the frequency of the subsequent DPF regenerations ifthe radial temperature gradient

$\left( \frac{\mathbb{d}T}{\mathbb{d}r} \right)$has exceeded a allowable temperature gradient

$\left( {{MAX}\frac{\mathbb{d}T}{\mathbb{d}r}} \right),$so that during each subsequent DPF regeneration event, less accumulatedsoot is burned off, thus reducing maximum temperature and maximumtemperature non-uniformity. The controller 208 may likewise decreaseexhaust oxygen concentration as by increasing EGR if the measured radialtemperature gradient

$\left( \frac{\mathbb{d}T}{\mathbb{d}r} \right)$is greater than a allowable temperature gradient

$\left( {{MAX}\frac{\mathbb{d}T}{\mathbb{d}r}} \right),$thus effectively decreasing the soot combustion rate or terminating theDPF regeneration, which in turn decreases temperatures and temperaturenon-uniformity inside the DPF. Another alternative measure is for thecontroller 208 to decrease late injection, which decreases the amount offuel available in the exhaust, thus helping to decrease the rate of orterminating the DPF regeneration. Further still, combinations of theabove approaches may be used.

Referring now to the specific example of the DPF overstress mitigationsystem 200 shown in FIG. 2, the system 200 is shown to include only twotemperature sensors, more particularly two radial temperature sensors204 positioned at 1 inch in from the exit face 206 of the DPF 202, withone positioned 1 cm in from the periphery 207 of the DPF and the other202 1 cm closer to a central axis (L) of the DPF. The controller 208 isconfigured to adjust the at-least-one engine operating parameter tocontrol the radial temperature gradient of the DPF near the exit face206 based on signals detected by the two near-exit-face temperaturesensors 204 and through the various actuators 212 coupled to thecontroller 208.

Referring now to the example DPF overstress mitigation system 200 shownin FIG. 3, the system 200 is shown here to include two exit-facetemperature sensors 204 and one deep temperature sensor. The twoexit-face temperature sensors 204 are positioned at 1 inch in from theexit face 206 of the DPF 202. One of the near-exit-face temperaturesensors 204 is positioned at 1 cm in from the periphery 207 of the DPFand the other is positioned 1 cm closer to a central axis (L) of the DPF202. The signal detected by the deep temperature sensor 214 is used tomodify a gradient signal detected by the two near-exit-face temperaturesensors 204. The controller 208 is configured to adjust the at-least-oneengine operating parameter to control the radial temperature gradient

$\left( \frac{\mathbb{d}T}{\mathbb{d}r} \right)$of the DPF near the exit face 206 based on the modified gradient signaland via the various actuators 212 coupled to the controller 208.

FIG. 4 is a flow chart illustrating an example method 400 for reducingoverstress of a DPF, which is implemented in a DPF overstress mitigationsystem according to the present disclosure. The method 400 includes:

At 402, measuring a deep temperature using a deep temperature sensorpositioned deeper into the DPF than the exit-face temperature sensors.In some examples, the DPF overstress mitigation system is provided witha deep temperature sensor. At 404, modifying a maximum allowable radialtemperature gradient near the exit face to be higher when the deeptemperature is lower than the near-end-face sensor at the same distancefrom the periphery. The maximum allowable radial temperature gradientnear the exit face with such conical equi-temperature contours isincreased from a maximum allowable stress of the DPF calculated forcylindrical equi-temperature contours, according to a calculation usingthe measured deeper temperature.

Again, before modification, the maximum allowable radial temperaturegradient near the exit face is based on the readings and locations ofthe exit-face temperature sensors that are used to measure the radialtemperature gradient near the exit face and stresses calculated assumingcylindrical equi-temperature contours.

In examples with deep-temperature sensing, the maximum allowable radialtemperature gradient near the exit face is beneficially modified whenthe measured deep temperature indicates that the equi-temperaturecontours are more-conical, rather than cylindrical. An example of suchmodification is discussed in reference to FIG. 3.

As noted above, in some examples, the maximum allowable radialtemperature gradient near the exit face itself varies with an operatingcondition of the diesel particulate filter and/or engine and otherfactors, as noted.

At 406, measuring an actual radial temperature gradient near the exitface of the DPF using at least two exit-face temperature sensors nearthe exit face of the DPF, the two exit-face temperature sensors beingradially positioned from each other.

At 408, adjusting at least one engine operating parameter duringregeneration of the DPF to maintain the radial temperature gradient nearthe exit face to below the maximum allowable radial temperaturegradient, as modified by the calculation including the deep temperature.

As described herein, various engine operating parameters may beadjusted.

FIG. 5 is a schematic diagram that illustrates approximately conicalequi-temperature contours formed within a DPF during filter regenerationwith peak temperatures pushed to the rear of the filter. As shown by aDPF equi-temperature contour 216 in FIG. 5, the DPF is hotter in thecentral rear region of the DPF and cooler along the periphery and frontof the DPF, with somewhat conical equi-temperature contours. Suchtemperature non-uniformity still causes the DPF to experience an axialstress which still raises the probability of micro-fracture or fractureof the honeycomb structure of the DPF, as described herein, compared toan unheated DPF. As better shown in FIG. 6, this axial stress 218 withconical equi-temperature contours is less in magnitude than forcylindrical equi-temperature contours with the same near-exit-faceradial gradient.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The specific routines described herein represent one or more of anynumber of processing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various acts,operations, or functions illustrated may be performed in the sequenceillustrated, in parallel, or in some cases omitted. Likewise, the orderof processing is not necessarily required to achieve the features andadvantages of the example embodiments described herein, but is providedfor ease of illustration and description. One or more of the illustratedacts or functions may be repeatedly performed depending on theparticular strategy being used. Further, the described acts maygraphically represent code to be programmed into the computer readablestorage medium in the engine control system.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,1-4, 1-6, V-12, opposed 4, and other engine types, such as gasolinedirect-injection, homogeneous charge compression ignition (HCCI), anddiesel, among others. The subject matter of the present disclosureincludes all novel and nonobvious combinations and subcombinations ofthe various systems and configurations, and other features, functions,and/or properties disclosed herein.

The following claims particularly point out certain combinations andsubcombinations regarded as novel and nonobvious. These claims may referto “an” element or “a first” element or the equivalent thereof. Suchclaims should be understood to include incorporation of one or more suchelements, neither requiring nor excluding two or more such elements.Other combinations and subcombinations of the disclosed features,functions, elements, and/or properties may be claimed through amendmentof the present claims or through presentation of new claims in this or arelated application. Such claims, whether broader, narrower, equal, ordifferent in scope to the original claims, also are regarded as includedwithin the subject matter of the present disclosure.

1. A method for reducing diesel particulate filter overstress,comprising: adjusting at least one engine operating parameter duringparticulate filter regeneration in response to a radial temperaturegradient near an exit face of the diesel particulate filter, the atleast one engine operating parameter including a frequency of subsequentdiesel particulate filter regenerations.
 2. The method of claim 1further comprising adjusting the at least one engine operating parameterto maintain the radial temperature gradient below a maximum allowableradial temperature gradient, wherein the maximum allowable radialtemperature gradient near the exit face corresponds to a maximumallowable stress of the diesel particulate filter for an acceptably lowprobability of failure less than a threshold.
 3. The method of claim 2,further comprising: measuring the radial temperature gradient near theexit face of the particulate filter from a plurality of radiallyseparated temperature sensors; measuring a deep temperature away fromthe exit face and deeper into the diesel particulate filter from a deeptemperature sensor; wherein the maximum allowable radial temperaturegradient near the exit face is modified based on the measured deeptemperature.
 4. The method of claim 2 further comprising adjusting theat least one engine operating parameter to maintain the radialtemperature gradient below a maximum allowable radial temperaturegradient, wherein the maximum allowable radial temperature gradientvaries with an operating conditions of the diesel particulate filter. 5.The method of claim 2, wherein adjusting responsive to the radialtemperature gradient further includes adjusting a throttle position. 6.The apparatus of claim 2, wherein adjusting responsive to the radialtemperature gradient further includes adjusting a late injection amount.7. The method of claim 1, wherein adjusting responsive to the radialtemperature gradient further includes adjusting exhaust gas flow to thediesel particulate filter.
 8. A diesel particulate filter overstressmitigation apparatus, comprising: two temperature sensors located in theexit face portion of the particulate filter, the sensors configured tomeasure a radial temperature gradient near the exit face of the dieselparticulate filter; and a controller configured to adjust at least oneengine operating parameter based on the radial temperature gradient,wherein the controller is further configured to adjust the at least oneengine operating parameter if the measured radial temperature gradientis greater than a maximum allowable radial temperature gradient, andwherein the maximum allowable temperature gradient is based on locationsof the two exit face temperature sensors.
 9. The apparatus of claim 8,wherein the maximum allowable radial temperature gradient varies withoperating conditions of the diesel particulate filter.
 10. The apparatusof claim 8, wherein the at least one engine operating parameter includesexhaust gas flow to the diesel particulate filter.
 11. The apparatus ofclaim 8, wherein the at least one engine operating parameter includesfrequency of the subsequent regenerations of the diesel particulatefilter.
 12. The apparatus of claim 8, wherein the at least one engineoperating parameter includes an exhaust oxygen concentration.
 13. Theapparatus of claim 8, further comprising a deep temperature sensorconfigured to measure a deep temperature that is deeper into the filterthan the two exit face temperature sensors; wherein the controller isfurther configured to adjust the at least one engine operating parameterbased on the measured radial temperature gradient and the measured deeptemperature.
 14. The apparatus of claim 13, wherein the maximumallowable radial temperature gradient near the exit face is modifiedbased on the measured deep temperature.
 15. A diesel particulate filteroverstress mitigation apparatus, comprising: two exit face temperaturesensors configured to measure a radial temperature gradient near theexit face of the diesel particulate filter; a deep temperature sensorconfigured to measure a deep temperature that is deeper into the filter;and a controller configured to adjust at least one engine operatingparameter if the measured radial temperature gradient is below a maximumallowable radial temperature gradient, the maximum allowable radialtemperature gradient being modified based on the measured deeptemperature.